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1
To make the dumplings, cut the fish into 1 1/2-inch chunks and place in the freezer for 10 to 15 minutes to freeze partially.
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2
Transfer to a food processor and process to as fine a puree as possible.
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3
Add the onion, egg, matzo meal, salt, and pepper and continue processing to a paste.
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4
Cover and set aside in the refrigerator until quite firm, at least 1 hour or up to overnight.
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5
To make the beet horseradish, cook the beet in water to cover until it can barely be pierced, 20 to 30 minutes, depending on the size.
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6
Drain and let cool until it can be handled, then peel it while still warm.
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7
Grate on the medium-fine holes of a box grater.
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8
In a small bowl, combine the grated beet, horseradish, vinegar, sugar, and salt and stir to mix.
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9
Set aside at room temperature until ready to use or refrigerate for up to 2 days.
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10
To make the broth, combine the fish bones and heads, carrot, celery, shallot, salt, and water in a medium saucepan, adding more water if needed to cover the ingredients.
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11
Bring to a boil over high heat and skim off any foam that forms on the surface.
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12
Decrease the heat to maintain a brisk simmer, cover partially, and cook until slightly reduced, about 30 minutes.
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13
Strain through a fine-mesh sieve into a large pot and bring to a simmer over medium-high heat.
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14
To cook the dumplings, with wet hands, form the fish mixture into ovals the size of a very large egg, using about 1/3 cup for each dumpling.
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15
Drop them into the broth and bring just to a boil.
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16
Cover the pot and simmer, adding more water if necessary to keep the dumplings afloat, until cooked through, about 1 1/2 hours.
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17
Transfer the dumplings and broth to a serving bowl, let cool until no longer steaming, and refrigerate until the broth sets into a loose gelee, 3 hours or so.
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18
Serve the gefilte fish and gelee cold with the beet horseradish on the side.
